Iron carbide formation on thin iron films grown on Cu(100): FCC iron stabilized by a stable surface carbide

Thin iron films evaporated onto Cu(1 0 0) were carburized using ethylene to produce iron carbide surfaces for use as model systems in experimental research. XPS and AES confirm that ethylene dissociation produces a pure iron carbide. A maximum of 0.5 ML carbon can be deposited for film thicknesses below 12 ML where Fe grows as gamma-iron (FCC). For thick, BCC-Fe(110) films, post-treatment with ethylene leads to carbon coverages beyond 0.5 ML where some carbon diffuses into the bulk. The film remains alpha-iron (BCC) and a different surface carbide with a (4 x 3) unit cell is found. On the thin FCC-Fe(100) films, carbon reconstructs the surface into a p4g(2 x 2)-Fe2C layer which has a special stability and acts as a carbon trap that prevents carbon diffusion into the bulk. Fe2C is thermally stable up to 700 K above which Fe diffuses into the copper substrate while leaving graphitic carbon behind. Carbon segregates to the surface during evaporation of iron on top of an Fe2C-covered FCC-Fe film and causes the film to retain the FCC structure up to a thickness of at least 30 ML, far beyond 12 ML where BCC-Fe forms on Cu(1 0 0) in absence of surface carbon.